Method of grain refining aluminum base alloys

ABSTRACT

THE INSTANT DISCLOSURE TEACHES A METHOD OF GRAIN REFINING ALUMINUM BASE ALLOYS. THE METHOD OF THE PRESENT INVENTION COMPRISES ADDING BORON AND TITANIUM TO THE MELT IN SPECIFICALLY DEFINED QUANTITIES.

United States Patent O Int. Cl. C22c 21/00 US. Cl. 75-142 Claims ABSCTOF THE DISCLOSURE The instant disclosure teaches a method of grainrefining aluminum base alloys. The method of the present inventioncomprises adding boron and titanium to the melt in specifically definedquantities.

BACKGROUND OF THE INVENTION Aluminum alloys can be classified based ontheir castability into two general classes, namely those which arestress sensitive and those who are nonstress sensitive.

Sound bar in nonstress sensitive alloy can be cast at high speeds byusing relatively large quantities of water. The as-cast macrostructureof these alloys is not especially important and sound bars can be castregardless of whether the macrostructure is columnar denritic orequiaxed.

Nonstress sensitive alloys do, however, frequently suffer from theformation of objectionable surface grain boundary cracks.

The stress sensitive alloys, however, present particular problems andspecial care must be taken in order to cast sound bars. Steeptemperature gradient which developed in the ingot during solidificationand cooling must be minimized in order to neutralize thermally inducedstresses that cause cracking of the bar. In addition, the macrostructureof the bar must be controlled by proper grain refining additions orother suitable means. When a fine equiaxed grain size prevails, soundbar can be produced over a wide range of casting conditions. However,when the structure is columnar denritic, it is virtually impossible tocast sound bar under any conditions.

Accordingly, it is a principal object of the persent invention toprovide a method of grain refining aluminum base alloys.

It is a further object of the present invention to provide a method ofgrain refining both stress sensitive and nonstress sensitive aluminumbase alloys.

It is a still further object of the present invention to provide amethod which is simple, convenient and easily practiced on a commercialscale.

Further objects and advantages of the present invention will appear fromthe ensuing specification.

SUMMARY OF THE INVENTION In accordance with the present invention it hasnow been found that the foregoing objects and advantages may be readilyobtained.

The present invention comprises a method of grain refining aluminum basealloys which comprises:

(A) Providing a molten aluminum base alloy material containing iron 0.04to 2%, silicon 0.02 to 2%, balance essentially of aluminum;

(B) Adjusting the melt to contain from 0.004 to 0.01% boron, preferably0.004 to 0.006% boron;

(C) adding titanium and additional boron to the melt so that 1) thefinal titanium content of the melt is from 0.01

(2) the final boron content of the melt is from 0.005 to 3,676,111Patented July 11, 1972 'ice (3) the titanium to boron ratio in the meltis from 1.4

to 2.2; and (4) the boron unbalance is between with the boron unbalancebeing determined by the foling relationship Boron unbalance=percentboron in the melt O.46 times percent titanium in the melt DETAILEDDESCRIPTION As indicated hereinabove, the present invention is a methodof grain refining aluminum base alloys containing iron from 0.04 to 2%and silicon from 0.02 to 2%. Magnesium may be present from 0 to 2%,preferably from 0.3 to 1.4% and copper from 0 to 1.53%, preferably from0.01 to 1.0%.

The present invention is particularly useful with respect to certainaluminum base alloys which will be described in more detail hereinbelow,such as especially Aluminum Alloy 6201, 5005 and with respect toelectrical conductor grade aluminum. The material which has beendesignated by the American Aluminum Association as electrical conductorgrade aluminum has a chemical composition of 99.45% minimum aluminum andan electrical conductivity of at least 61% IACS. The conductivitypercentages were established according to the American Society forTesting Materials and are based on an equal volume of the InternationalAnnealed Copper Standard (IACS).

Particularly preferred aluminum base alloys which are utilized inaccordance with the present invention contain silicon in an amount from0.02 to 1.3%, optimally from 0.2 to 1.3%, iron from 0.04 to 1%,optimally 0.1 to 0.7%, and copper from 0.01 to 1% and optimally from0.05 to 0.4%.

The alloys which are preferably utilized in accordance with the presentinvention preferably also contain magnesium in an amount from 0.3 to1.4%, they may also contain manganese 0.8% max., chromium 0.35% max.,zinc 0.5% max., and others 0.05 each, total 0.15%.

Aluminum Association Alloy 6201, which has an electrical conductivity ofat least 52.5% IACS, is a stress sensitive alloy and presents particularcasting problems. It has been found that this material respondsparticularly well to the method of the present invention and is aparticularly preferred material in the process of the present invention.The chemical composition of this material as listed with the AluminumAssociation is as follows:

6201 Aluminum Elements: Alloy, percent (1) Copper 1 0.10 (2) Iron 1 0.50(3) Silicon 0.50-0.90 (4) Manganese 1 0.03 (5) Magnesium 0.60-0.90 \(6)Zinc 1 0.10 (7) Chromium 1 0.03 (8) Boron 1 0.0 6 (9) Other elements,each i 1 0.03 (10) Other elements, total 1 0.1 (11 Aluminum RemainderMaximum.

Alloy 5005, which is also a particularly preferred material, contains:iron, 0.70% max.; silicon, 0.40% max.; copper, 0.20% max.; manganese,0.20% max.; magnesium, 0.5 to 1.1%; chromium 0.10% max; zinc, 0.25%max.; others, 0.05% each, total, 0.15%; aluminum balance.

In accordance with the present invention, the particular aluminummaterial is provided in molten form.

A small boron addition is made so that the boron content of the melt isfrom 0.004 to 0.006%, nominally 0.005%. This addition is made in themelting or holding furnace prior to casting. The particular object ofthis initial boron addition is to precipitate any chromium, zirconium,vanadium or titanium present as insoluble borides since any of theseelements in solid solution will reduce the electrical conductivity ofaluminum and aluminum base alloys. If the particular melt alreadycontains boron, this addition is not necessary. In accordance with theinstant process, one can tolerate up to 0.01% boron initially in themelt.

The next step in the process of the present invention is the addition tothe melt of titanium and boron in a particular relationship. Thetitanium and boron addition must be such that all requirements of thisrelationship exist in order to obtain proper grain refining effect.

The first requirement of the addition is that the final titanium contentof the melt be from 0.01 to 0.05%.

The second requirement is that the final boron content of the melt befrom 0.005 to 0.02%.

The third requirement is that the titanium to boron ratio is from 1.4 to2.2.

The fourth requirement of the titanium and boron addition is that theboron unbalance in the melt be between The boron unbalance is determinedby using the following relationship:

Boron unba1ance=percent boron in the melt 0.46 times percent titanium inthe melt The titanium to boron ratio and the boron unbalance in the meltare related theoretically and are particularly significant. One reasonfor the particular criticality of these values in the instant process isto prevent the electrical conductivity from being reduced by thepresence of titanium which would be in solid solution. Since thestoichiometry for TiB is 2.2. to 1, in order to insure that no titaniumis in solution, we prefer to be at this ratio or slightly on the excessboron side. In addition, if the foregoing requirements are met, we findthat maximum grain refining is obtained.

The manner of making the titanium and boron additions is notparticularly critical and any method known in the art may be utilizedfor making these additions. For example, the titanium and boronadditions may be made by continuously feeding an aluminum-5% titanium-1%boron alloy wire into the melt at a controlled rate as the melt passesthrough the transfer trough onto the casting.

Naturally, precautions should be exercised during casting to minimizethermally induced stresses, such as in accordance with US. patentapplication S.N. 110,938 for Continuous Casting Process For StressSensitive Aluminum Alloys by J. E. Dore and W. O. Staufier, filed Jan.29, 1971.

In accordance with the present invention it has been found that theprocess of the present invention obtains remarkable grain refiningwithout sacrifice of electrical conductivity particularly with respectto the stress sensitive aluminum base alloys. Cracking in the bars hasbeen found to be virtually eliminated, which has been a particularlytroublesome problem heretofore.

The nonstress sensitive alloys are also greatly improved. Thesematerials advantageously do not form surface grain boundary cracks dueto the provision of a much finer surface grain by the process of thepresent invention. By converting from columnar dendritic to equiaxedmacrostructure, the hot rolling characteristics are improved byminimizing grain boundary separation. With respect to the nonstresssensitive alloys also, these improvements are obtained without sacrificeof electrical conductivity.

The present invention and the advantages therefrom will be more readilyunderstandable from a consideration of the following illustrativeexamples.

Example I Commercial grade aluminum pig was charged to a holding furnaceand melted. The melt temperature was raised to about 1380 F.Subsequently, sufficient quantities of silicon (added as an aluminum-50%silicon master alloy), magnesium (added as magnesium pig) and iron(added as alumiunm-35% iron master alloy) were added to produce an alloyof the following nominal composition; magnesium 0.65%, silicon 0.60% andiron 0.20%. On analysis, the boron content of the melt was found to be0.002% instead of the desired nominal level of 0.005%. Hence, additionalboron as an aluminum-3% boron master alloy was added to the holdingfurnace to adjust the boron level. The melt was stirred for 10 minutesand then allowed to cool to a temperature of about 1340 F. At thispoint, a composition check showed that the melt contained 0.005% boronand 0.001% titanium.

The tap hole of the holding furnace was open, the melt allowed to flowto the direct chill casting unit and casting started. Shortly after thestart of casting, samples taken from the cast bar revealed a coarsecolumnar dendritic macrostructure and cracks in the center of the bar.Subsequently, additional titanium and boron were added to the flowingmetal stream in the transfer trough by continuously feeding A" diameteraluminum-5% titanium- 1% boron wire at a rate to add about 0.02%titanium. Analysis of the cast bar showed 0.021% titanium and 0.0105boron. Under these conditions, the titanium to boron ratio was 2.0 andthe boron unbalance was 0.0105 -0.46 0.021 or about +0.00l%. Theresultant cast bar produced under these conditions had a fine equiaxedmacrostructure and was free of cracks and was internally sound.

Example II Example I was repeated using the same alloy and procedure asdescribed except that the intial boron content of the melt wasdetermined to be 0.003% and an addition of 0.004% boron was made to themelt as an aluminum-3% boron master alloy. At this point, the boroncontent of the melt was 0.007% and the titanium content of the melt was0.001%. During casting, additional titanium and boron was added to theflowing molten metal stream in the transfer trough continuously in theform of aluminum-5% titanium-1% boron wire as in Example I to add 0.015%titanium.

The final boron content of the alloy was 0.010% while the titaniumcontent was 0.015% as determined by analysis. This provided a titaniumto boron ratio of 1.5 and a boron unbalance of 0.0l0-0.46 0.015 or about+0.003%. As in Example I, the resulting cast bar had a fine equiaxedmacrostructure and was internally sound and free of cracks.

Example 111 Example I was repeated with the same alloy, except that theinitial melt sample showed 0.01% boron. No boron addition was made tothe melt. During casting, an addition of aluminum-5% titanium-1% boronwire was continuously fed into the flowing metal stream in the transferthroughs at a rate to add 0.01% titanium. Analysis of cast bar samplesshowed 0.01% titanium and 0.012% boron present. This gave an unfavorabletitanium to boron ratio of 0.83 and an unfavorable boron unbalance of0.0l20.46 0.01 or about +0.0074%. As expected, the resultant bar had acoarse columnar dendritic macrostructure and contained numerous andsevere internal cracks.

Example IV Example III was repeated except that additional aluminum-5%titanium-1% boron was added to the flowing molten metal stream in thetrough as wire so as to provide a total titanium content of about 0.04%.The new conditions yielded a final composition containing about 0.035%titanium and 0.017% boron. The titanium to boron ratio was now desirably2.1 and the boron unbalance was 0.0170.46 0.035 or about +0.001%. Inthis case, the resultant bar had a fine equiaxed macrostructure and wasinternally sound and free of cracks.

Example V Commercial grade aluminum pig was charged to a holding furnaceand melted. The melt temperature was raised to about 1380 F.Subsequently, sufficient copper (added as metallic copper) and iron(added as aluminum-25% iron master alloy) were added to produce an alloyof the following nominal composition; iron 0.65% and copper 0.40%. Onanalysis, the boron content of the melt was found to be less than0.001%, instead of the desired nominal level of 0.005%. Hence, anaddition of 0.005% boron as an aluminum-3% master alloy was made to theholding furnace to adjust the boron level. The melt was stirred forminutes and then allowed to cool to a temperature of 1340 F. At thispoint, a composition check showed that the melt contained 0.005% boronand 0.001% titanium. The tap hole of the holding furnace was opened, themelt allowed to flow to the direct chill casting unit and castingstarted. Shortly after the start of casting, samples taken from the castbar revealed a coarse columnar dendritic macrostructure. In addition,numerous cracks were detected in the surface of the bar by means of dyecheck analysis. Subsequently, additional titanium and boron were addedto the flowing metal stream in the transfer trough by continuouslyfeeding diameter aluminum-5% titanium-1% boron wire at a rate to addabout 0.02% titanium. Analysis of the cast bar showed 0.019% titaniumand 0.009% boron. Under these conditions, the titanium to boron ratiowas 2.1 and the boron unbalance was +0.0003% Cast bar produced underthese conditions had a fine equiaxed macrostructure and the surface ofthe bar was free of cracks and other defects.

This invention may be embodied in other forms or carried out in otherways without departing from the spirit or essential characteristicsthereof. The present embodiment is therefore to be considered as in allrespects illustrative and not restrictive, the scope of the inventionbeing indicated by the appended claims, and all changes which comewithin the meaning and range of equivalency are intended to be embracedtherein.

What is claimed is:

1. A method of grain refining aluminum base alloys which comprises:

(A) providing a molten aluminum base alloy material containing iron from0.04 to 2%, silicon from 0.02 to 2%, balance essentially aluminum;

6 (B) adjusting the boron content of the melt to provide from 0.004 to0.01% boron in the melt; (C) adding titanium and additional boron to themelt so that (1) the final titanium content of the melt is from 0.01 to0.05%; (2) the final boron content of the melt is from 0.005 to 0.02%;(3) the titanium to boron ratio is from 1.4:1 to

2.2: 1; and (4) the boron unbalance is between with the boron unbalancebeing determined by the following relationship Boron unbalance=percentboron in the melt 0.46 times percent titanium in the melt (D) castingthe said melt.

2. A method according to claim 1 wherein the boron content of the meltis adjusted to contain from 0.004 to 0.006% boron in the melt.

3. A method according to claim 2 wherein said aluminum material is astress sensitive material.

4. A method according to claim 2 wherein said material is a nonstresssensitive material.

5. A method according to claim 1 wherein said aluminum base alloy iselectrical conductor grade aluminum.

6. A method according to claim 1 wherein said aluminum material is analuminum base alloy containing silicon from 0.02 to 1.3%, iron from 0.04to 1%, copper from 0.01 to 1%, balance essentially aluminum.

7. A method according to claim 6 wherein said alloy contains magnesiumfrom 0.3 to 1.4%

8. A method according to claim 6 wherein said alloy contains manganese0.8% max., chromium 0.35% max., zinc 0.5% max., others, 0.05% each,total, 0.15%, balance essentially aluminum.

9. A method according to claim 1 wherein said aluminum base alloy isAlluminum Alloy 6201.

10. A method according to claim 1 wherein said aluminum base alloy isAluminum .Alloy 5005.

References Cited UNITED STATES PATENTS 6/1933 Nock -138 8/1933 Bonsack75-138 RICHARD O. DEAN, Primary Examiner

